The number of mobile devices or user nodes desiring service from wireless communication systems is expected to increase dramatically in the coming years, and the amount of control information used to coordinate and control these user nodes will increase proportionally. Control channels, which are predetermined or agreed upon radio resources used for exchanging control information, reduce the radio resources available for data transmission. Thus it is desirable to optimize or minimize the amount of radio resources used for control information within a wireless communication network.
Conventional wireless communication systems based on current standards, such as Long Term Evolution (LTE), employ non-precoded control channels. This results in a single data stream in each element of a downlink control channel targeted to a particular user node or broadcast to a group of user nodes. For example, in LTE, the downlink control data is appended, prior to transmission, with a cyclic-redundancy check (CRC) that has been scrambled with the user node identity. This allows a user node to identify its own data without the need for additional payload. However, this scheme does not allow exploiting a single time-frequency radio resource to send multiple control data streams to different user nodes.
Multiple-input multiple-output (MIMO) systems, where either or both of the access node and user node are equipped with multiple antennas, make it possible to multiplex multiple data streams over the same time-frequency or time-frequency-code radio resources by exploiting the multiple transmissions paths, or layers, provided by the MIMO systems. Typical multi-user MIMO schemes rely on accurate and timely channel state information (CSI) at the transmitter (CSIT) to achieve the benefits of MIMO transmission. Accurate CSI is achieved by channel sounding or transmitting reference signals, often referred to as pilot symbols or pilots, interleaved with the data so the receiver can measure and estimate the channel. The pilots need to be sent often enough to account for changing channel conditions to allow reliable decoding of data at each receiver. Also, the CSI, or CSIT needs to be acquired across the full allocated frequency band of the downlink data transmission for the channel estimation to be accurate. Repeatedly sending pilot symbols across the full frequency band requires significant radio resources. The requirements in terms of transmit power scale with bandwidth while the requirements in terms of radio resources used for the uplink pilot symbols scales with the number of user nodes. Thus the number of user nodes that can be served by an access node may be limited by the available sounding resources and control channel capacities.
Thus there is a need for improved methods and apparatus for managing and transmitting control information in wireless communication systems.